1. Field of the Invention
The present invention pertains to subsurface conversion mining of solid substances, such as coal, lignite, minerals, and the like. Such substances are typically deposited in a stratified earth formation wherein the substance in question forms one seam or stratum, or a portion of a stratum. Of the large number of such deposits existing in the earth, only a relatively small percentage are minable by conventional manual or mechanical mining techniques.
There are several reasons for the unminability of the majority of these deposits. One of these is that conventional mining techniques are typically designated either to mine deposits which are relatively close to the earth's surface, or to mine very deep deposits. Thus, such techniques are not suitable for mining deposits located at intermediate depths. Likewise, most excavation-type mining techniques are designated for use in seams of an intermediate thickness, and are not practical for mining of extremely thick or thin seams. Still other deposits are unminable because they are located beneath cities, and conventional mining of these deposits would pose too great a danger to the structural integrity of the buildings, streets, sewers, etc. of these cities. In still other areas, such prior mining techniques are objected to because of the ecological damage which they may produce.
2. Description of the Prior Art
Accordingly, techniques have been developed for subsurface mining of such deposits by chemical conversion techniques (or unit processes) and/or physical conversion techniques (or unit operations). In such processes, conversion media may be injected into the deposit to be mined through a relatively small well which does not require extensive excavation at the earth's surface. The conversion media converts the solid substance to be mined into a flowable form. For example, in chemical processes combustion supporting media may be injected to burn the solid substance and convert it to a gas. In physical processes, the conversion may involve the injection of a liquid medium which either dissolves the solid substance to be mined or breaks it up forming a slurry or suspension. In any event, the subsurface conversion of the solid substance into a flowable form permits it to be withdrawn from the subsurface stratum by a relatively small well, which again does not require extensive excavation or clearing of the land at the earth's surface.
Such subsurface conversion mining techniques have offered some improvements over the aforementioned mechanical mining processes in that they are not as limited in terms of the depths or thicknesses of seams in which they may be used. However, the prior art conversion techniques have in turn created their own problems. One of these is that, when the deposit to be mined is extremely compact and/or substantially monolithic in form, problems arise in causing the conversion media to properly permeate the formation.
Another major problem in many prior conversion processes is that the conversion may be virtually uncontrolled, both in terms of the direction in which the conversion proceeds and the extent of the deposit which is so converted. This problem is particularly acute in chemical conversion in the form of burning or oxidation. Where such burning is uncontrolled, either in extent or direction it may not only be wasteful and inefficient, but may also lead to collapse of the overburden. Another factor which may contribute to such collapse is the failure of various prior conversion mining processes to provide for proper support of the overburden, regardless of whether or not the conversion proceeds in an uncontrolled manner.
Still another problem with the prior techniques is that they may permit fluids, which might be comprised of the conversion media and/or the flowable products of the conversion process, to pass into strata of the earth formation other than the one being mined. This in turn can lead to contamination of underground supplies of water or other fluids, and in some cases, dangerous gases may even be permitted to pass directly into the earth's atmosphere via a permeable stratum communicating obliquely with the earth's surface. Conversely, the prior processes also generally fail to prevent foreign gases and/or liquids already present within the earth formation from passing into the area being mined and thereby interfering with the conversion process and/or contaminating the products.